The present invention relates to integrated semiconductor circuits and, more particularly, to a semiconductor die having a power supply de-coupling capacitor.
A semiconductor die includes a silicon substrate and a pattern of semiconductor devices, such as transistors, resistors and diodes, which are fabricated on the substrate. The devices are electrically interconnected by one or more segments of conductive material which extend along routing layers applied over the top of the semiconductor devices. The conductive segments on one routing layer are electrically coupled to conductive segments on other layers through conductive vias.
Power supply conductors supply power to the individual devices on the die. These power supply conductors are fed by one or more main power supply buses which are typically routed on one of the routing layers along the periphery of the die. An external power supply is then coupled to the power supply buses to provide power to the die.
Each transistor on the die has an interconnect capacitance at its output. When a transistor on the die changes its output state, the transistor either sinks current from the external power supply to charge the interconnect capacitance or sources current to the external power supply to discharge the interconnect capacitance. In essence, the interconnect capacitance at the output of the transistor shares charge with a capacitance in the power supply.
Due to the large distance between the power supply and the transistor, the charge sharing between the power supply and the transistor output is relatively inefficient and generates noise in the rate of change of current at the output with respect to time. This noise has been suppressed to some extent in devices of the prior art by physically coupling an external capacitor across the power supply inputs to the die. However, this method of suppressing noise is also inefficient since the capacitor is still a large distance from the individual transistors in which it shares charge.